feat(storage): media content-addressable cache and monitoring base64 externalization

- Add MediaCache using xxHash3-128 (with sha256 fallback) content-addressable storage
- Externalize message chain image payloads before recording monitoring and discarded messages
- Strip base64 payloads to null in SQLite monitoring_messages, dropping row size from megabytes to hundreds of bytes
- Add GET /api/v1/files/media/<filename> route with immutable HTTP cache headers to serve cached media
- Integrate age-based retention (default 30 days) and configurable disk quota with MaintenanceService cleanup loop
- Add defensive sanitizer in MonitoringService.record_message against oversized raw base64 payloads
- Add comprehensive unit tests and end-to-end verification covering CAS deduplication, route serving, and LRU pruning

pyproject.toml

@@ -58,6 +58,7 @@ dependencies = [
     "python-docx>=1.1.0",
     "pandas>=2.2.2",
     "chardet>=5.2.0",
+    "xxhash>=3.5.0",
     "markdown>=3.6",
     "beautifulsoup4>=4.12.3",
     "ebooklib>=0.18",